The invention relates to a method for creating through-connections in holes in multi-layer printed circuit boards wherein the circuit board is placed on a supporting frame which leaves the area beneath the holes free, and conductive paste is injected into the holes in the board by a dispensing means with the formation of overlaps, such that a protruding, outwardly spreading plug is formed on the back of the holes.
Published German Patent Application No. DE 3,509,626, relating to making through-connections in printed circuit boards, discloses the application of conductive paste to the holes in the circuit boards by means of a screen printing process, then drawing the conductive paste partially into the holes by applying a vacuum beneath the board while preventing the conductive paste from reaching the bottom of the boards. Then the circuit boards must be laid in the apparatus with their underside up, and the described procedure of introducing conductive paste into the holes in the boards is repeated. A through-connection is thus created, but it is disadvantageous that the printed circuit board always must be placed in the apparatus a second time, and that a spot of conductive paste of varying thickness, but comparatively very thick overall, is created in the area of the holes and in the area of the margins thereof. Furthermore, the diameter of the holes is not precisely defined after the connection is completed.
Published European Patent Application No. EP 194,247 proposes that through-connections be made in circuit boards by applying conductive paste by means of a screen printing process with the formation of marginal overlaps and then drawing the conductive paste into the bores by means of a vacuum so that the conductive paste exits from the back side. Subsequently, the conductive paste is partially removed by compressed air with the formation of marginal overlaps on the back, while conductive paste also remains on the walls of the holes. This method, however, requires a complex printing machine. Also, a template as well as a matching bottom support must be prepared. Consequently, this process is economical only for large scale production. U.S. Pat. No. 5,145,691 also discloses filling the holes of printed circuit boards with conductive paste by means of a dispensing device in order to make through connections. The conductive paste then remains in the hole and fills it up completely. Additionally, it is necessary in this case to apply a mask over the circuit board.